Name_________________
Series and Parallel Resistance.
A series circuit consists of resistors connected to the power supply in such a way
that current flows through each resistor in turn, one after another. The
Equivalent resistance of a series circuit (R) is equal to the sum of all of the
resistances in the circuit.
R = R1 + R2 + R3…….
The potential difference (V) across the voltage source is equal to the sum of
voltage drops across each resistor.
V = V1 + V2 + V3…….
The total current flowing through the circuit is most easily found by calculating the
equivalent resistance and then applying a form of Ohm's Law:
I=
V
R
Where I is the current in amperes, R is the equivalent resistance, and V is the
voltage source.
You will need:
• A voltmeter
• An ammeter
• Several wires with clips
• 3 different resistors
• A switch
• A power supply
Procedure:
For the first part of this lab, you will set up a series circuit 3 times, adding a
resistor each time. By now you should know how to set up a circuit, read a circuit
diagram and correctly connect the meters. See the appendix if you have any
questions.
Series Circuits:
Circuit 1:
This first circuit is identical to the one that you did in the Ohm's law lab. Set the
power supply to 3 volts and be sure to close the switch only long enough to take
readings, to avoid burning the resistor. Enter your readings on the data table.
Circuit 2:
Connect the second resistor in series with the
first and measure the voltage drop across both
resistors together. Record all observations in
the data table.
Circuit 3:
Add a third resistor in series with the other two, and set the voltmeter up to
measure the voltage drop across all of the
resistors together. After recording all values,
move the voltmeter connections to measure
the voltage drop across each resistor
individually.
While you are doing your series circuits, I will set up a parallel circuit in the front
of the room to save time and scarce equipment, and I will call groups up
individually to do the parallel circuit part of the lab.
Parallel Circuit:
In a parallel circuit, the equivalent resistance is determined by the equation:
1
1
1
1
........
=
+
+
R R1 R2 R3
The total current (I) is equal to the sum of the currents through each branch of
the circuit.
I = I 1 + I 2 + I 3 ......
For this circuit, measure all resistances, the total current, and the current through
each branch and record them on the data table. You will need to move the
ammeter to measure the individual currents one at a time. Also measure and
record the voltage drop across each resistor.
Data Tables:
Series
Circuit 1
R1 (Ω)
R1 (Ω)
Circuit 2
R1 (Ω)
Circuit 3
R2 (Ω)
R2 (Ω)
R3 (Ω)
Ammeter
reading (A)
Voltmeter
reading (V)
Ammeter
reading (A)
Voltmeter
reading (V)
Ammeter
reading (A)
Voltmeter
readings (V)
V V1 V2 V3
Tolerance (%)
Parallel
R1
( Ω)
R2
( Ω)
R3
( Ω)
Ammeter readings (A)
I
I1
I2
I3
Voltmeter readings (V)
V
V1
V2
V3
Tol.
(%)
Questions:
For all questions, you must show all work. (no work = no credit)
1. For the third series circuit, use the voltage and current data to calculate
a) The equivalent resistance
b) The resistance of each resistor.
Do they agree within the tolerance ranges of each resistor?
2.For the third series circuit, calculate the total voltage drop from V1, V2, and V3.
Does this agree with the measured total drop? If not, Why?
3. What determines the total current in a series circuit?
4. For the parallel circuit, calculate the current through each branch.
5 For the parallel circuit, calculate the resistance in each branch, and from that,
the equivalent resistance.
6. For the parallel circuit, calculate the total current from the current in each
branch. Does this agree with the measured total current? If not, why?
Application:
A set of 50 Christmas lights of equal resistances is running in a 120 Volt power
source. The current running through the circuit is 1.0 A. What is the resistance of
each bulb if:
a) the circuit is a series circuit?
b) the circuit is a parallel circuit
Appendix
Use of the Meters:
• Voltmeter
The voltmeter is used to determine the difference in electric potential between
two points in a circuit. It is always connected in parallel, as indicated in the
circuit diagram, never in series (it can't measure potential difference across
itself). If you can disconnect the voltmeter without interrupting the circuit, you
have connected it properly. The positive terminal (+) of the voltmeter should
be connected to the positive side of the power supply, either directly or
through components. The negative terminal should be connected to the
negative side, either directly or through components. The voltmeters we have
in the lab only go up to 3 volts, so do not use a setting on the power supply
higher than that.
•
Ammeter
The Ammeter measures the amount of current in a circuit and MUST be
connected in series. Connecting the ammeter in parallel will damage the
instrument. The circuit must be interrupted when you disconnect the
ammeter. The positive terminal should be connected either directly or
through circuit components to the positive end of the power supply. The
negative terminal should be connected either directly or through components
to the negative end of the power supply. There are two different ammeters
available in the lab, one ranging from 0.0 to 1.0 A in 0.1 a increments, and
one ranging from 0.0 to 10.0 A in 1 A increments. If you are having trouble
getting a reading, either the ammeter does not seem to be registering a
current or it is too small to read, or it is "pinned" at the high end of the range,
you may want to try a different one, depending on the resistance in the circuit.
Ohm's law states that given a certain resistance (R), the electric current (I)
flowing through a circuit is directly proportional to the applied voltage (V) and
inversely proportional to resistance.
R=
V
I
Resistance is measured in ohms (Ω) where 1 Ω=
1V
1A